1. Field of the Invention
This invention relates in general to automatic well test systems for sequentially testing individual wells in an oilfield. In particular, the invention relates to the automatic sequential testing of a plurality of producing wells through the use of a stand alone microprocessor based control system adaptable for various kinds of equipment used in oil fields across the world.
2. Description of the Prior Art
Oil as it is produced from the well normally contains not only oil but various amounts of gas and water as well. Wells also produce debris such as sand and the like in addition to oil, gas and water. Multiple wells are usually connected to a "production station", where the water and gas are separated from the oil. It is usually necessary to determine the amount of oil, gas and water produced on a well by a well basis. However, rather than provide separation and measurement devices for each well, the wells are commonly switched to a single test separator for individual measurements of the wells. The sequential testing of each of the wells in the field is known as "well test" and the cycling of two or more of the wells is known as the "well test sequence".
In a typical production station, each well is brought in on its own flow line to a diverter valve or valves which then route the flow to either a common production header or to a test header. A "header" is simply a pipe in which the flow from any well connected to it can be combined through diverter valves with the flow from all other wells on the header. In a typical production station only one well is diverted to the test header for a well test, while all other wells are diverted to the production header. The flow is then sent to the corresponding separator which may be the test separator corresponding to the test header or to the production separator corresponding to the production header. A separator is a vessel in which the oil/gas/water mixture "rests" for a retention time in order to allow natural separation of the oil, the gas and the water. The gas will "break out", the water will settle to the bottom and the oil will "float" on the water. By tapping the vessel at various levels, the three production components of the well can be selectively retrieved.
The top vent of the separator releases the gas from the well. A middle tap produces oil and a bottom tap produces water. The components are then "metered", and sometimes pressure and temperature compensated, to determine the composition of the well's production. Since some of the water will remain emulsified in the oil, not all of the water produced by the well will be recorded by the water line from the separator. For this reason, the oil of the oil line is referred to as the "oil emulsion" and a measurement in addition to its flow rate is made on the oil emulsion to determine its water content.
Prior art devices have been used to determine the water/oil content of the oil emulsion flowing in the oil outlet by measuring the capacitance of the fluid. Combining that measurement with a flow rate measurement in the oil outlet enables a determination of the net oil and emulsified water flow rate from the well.
A test separator is generally required in order to individually determine the flow characteristics of individual wells, because even though measurements are also performed at a production separator, the flow through the production separator comprises the combined flow of multiple wells. Individual well information is only available by sequentially selecting one well at a time to be routed through the test separator.
After metering and the net oil measurement for each well, the outputs from the free water line and the oil line are usually recombined and routed to the production header for normal production with the remaining wells. By providing the outputs of the test separator back to the production separator, the measurement devices on a production separator represent the "total" production quantities.
Many different configurations of equipment are possible for the test separator. Valving varies from one system to another. Multiple or single separators may be used for the test separator. Instrumentation on the separators varies from one oilfield to another and from one equipment manufacturer to another. Flow rates are different from well to well. The components of the produced well fluid differs from well to well as does the grade of the oil produced.
Still another variation in the kinds of test separators setups in oil fields relates to whether or not two phase or three phase flow is produced. As described above, a test separator having a free water outlet is typical of a "three phase" separator. In areas where water content is low enough that it remains emulsified and does not separate easily from the oil, the flow is referred to as "two phase". Two phase separators do not use internal baffels and weirs to separate free water from the oil emulsion as do three phase separators.
Prior art automatic well test systems have used relay controlled systems for sequentially switching the diverter valves of the wells to the test header for individually testing the wells. Such systems have been inflexible in that each system had to be designed individually for a particular well system. Such systems required individual design because of the large variety of physical equipment associated with test separators used in oil fields across the world. For example, turbine meters or positive displacement measuring devices for measuring flow in the various flow lines could be alternatively used.
Although central supervisory systems have been used in the past with remote terminal units in the field, no stand-alone programmable automatic well test unit has been provided before the invention to the described below.
One of the problems associated with prior automatic well test systems has been that there has been no convenient way to automatically test whether or not the diverter valves are leaking.
Still another problem has existed in prior automatic well test systems in that the purging of the test separator, prior to the test of an individual well, in order to insure that all of the produced fluid from a previous well test has been removed from the separator, has been controlled as a function of purging time. Although purging as a function of time may work satisfactorily where a well is producing at a constant rate, such purging which ordinarily lasts for a relatively long period of time, may be ineffective and inaccurate where a well produces cyclicly.
Still another problem with prior art automatic well test systems has related to the inability to automatically determine whether or not dump valves associated with a test separator are leaking or whether or not they are stuck open.